This document presents the development of a methodology for detecting rough road using the frequency analysis of the instantaneous engine speed signal. Combustion diagnosis (misfire detection) is enforced by EOBD/OBD II requirements, and eliminating the rough road driving affect on the diagnosis is critical for getting a robust misfire detection performance, especially for crankshaft speed based misfire diagnosis method. Although some kind of sensor is often used to identify that vehicle is traveling on rough road, the sensor less rough road monitor method is still more attractive for OEM to reduce cost, and some approaches have been developed past for the aim. The new approach presented here is based on the frequency domain analysis of engine speed characteristic excited by rough road traveling, and thus the frequency energy distribution comparison with public road driving and misfire introduced operation condition. A filter is then designed to process the engine speed signal based on the analysis result, and an index is calculated from the filtered signal and its unfiltered version. Once the index exceeds calibrated threshold, rough road driving is assumed present. This method had been built in ECU (Engine control Unit), and validated on a MPV vehicle in rough road test field.